1. * GB780050 (A)
Description: GB780050 (A) ? 1957-07-31
Travelling wave tubes
Description of GB780050 (A)
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The EPO does not accept any responsibility for the accuracy of data
and information originating from other authorities than the EPO; in
particular, the EPO does not guarantee that they are complete,
up-to-date or fit for specific purposes.
PATENT SPECIFICATION
78o0,o50 Date of Application and filing Complete Specification: April
29, 1955.
No. 12511/55.
Application made in United States of America on May 3, 1954.
Complete Specification Published: July 31, 1957.
Index at acceptance:-Classes 36, El; 39(1), D9(A: C: E: F: H), D10D,
D16A(l: 2), D(18A: 46A); and 40(8), U18(A1: A2: A5: B1).
International Classification:-C03c. HOlj. H03h.
COMPLETE SPECIFICATION
Travelling Wave Tubes We, STANDARD TELEPHONES AND CABLES LIMITED, a
British Company, of Connaught House, 63 Aldwych, London, W.C.2,
England, do hereby declare the invention, for which we pray that a
patent may be granted to us, and the method by which it is to be
performed, to be particularly described in and by the following
statement:-

2. The present invention relates to travelling wave tubes having a helix
mounted within a ceramic envelope and is more particularly concerned
with arrangements for coupling radio frequency energy between the
helix and input and output coaxial line feeders.
To, achieve the desired energy coupling in an efficient manner, the
impedance of the helix must be matched to, the input or output feeder
as the case may be. In the past, part of the feeder together with the
inmpedance matching section, has been disposed internally of, or as a
part of, the vacuum closure. Thisi latter structure requires the
inclusion of glass beads within the coaxial line to provide the
necessary vacuum seal. This work is tedious and the glass beads tend
to crack causing a leaky tube. Not only is the fabrication of such
elements time consuming and expensive, but if the tube becomes
defective during processing due to some vacuum tube fault,
non-emissive cathode or similar difficulty, the coaxial line feeder
and the matching section were also lost with the rejected tube,
thereby considerably increasing the cost of the good tubes produced.
The coupling means of this invention enables the coaxial line feeder
and its junction with the impedance matching section to be external to
the vacuum envelope which provides ease in final adjustment when
necessary and is a large cost reducing factor. The cost reduction is
brought about structurally as indicated above and in addition by a
reduction of time in the outgasi process of a given discharge device.
Reduction in time for outgassing a discharge device is accomplished by
decreasing the metal components within the vacuum enclosure.
[5 thereby eliminating material having a great affinity for the
entrapment of unwanted gases and by enabling an increase of the
outgassing temperature due to the employment of ceramic materials in
the fabrication of the tubes. 50 According to the present invention
there is provided a travelling wave tube comprising a tubular ceramic
envelope bonded at each end to, a respective transverse ceramic disc,
a helix supported within the enclosure of the said 55 envelope and the
ceramic discs, coaxial line input and output feeders external to the
said ceramic envelope and a dielectric loadedstripline
impedance-transforming coupling between each end of the helix and the
respective input 60 or output feeder, each, said coupling being formed
by a ground plane conductor on the outer face of a respective one of
the ceramic discs joined to the outer conductor of the adjacent said
feeder and a radial conducting 65 strip, providing the line conductor,
bonded to the inner face of the same said disc and connected at its
respective ends to' the adjacent end of the helix and to' the inner
conductor of the respective said feeder. 70 By a "strip-line" we mean
a transmission line in, the form of a relatively narrow strip' (the
line conductor) running parallel to and separated by dielectric from a

3. ground plane conductor of much greater width but which is 75 not
necessarily itself a strip. The line conductor may be tapered to
improve impedance matching between the helix and the coaxial line
feeder.
In embodiments of the invention: in which 80 an electron gun is
enclosed within a tubular ceramic extension of the envelope of the
travelling wave tube, the extension may be closed by a ceramic disc
instead of the usual glass press and leads to the electron gun
electrodes may 85 be brought out by means of radial conducting strips
bonded to, the last mentioned ceramic disc.
Embodiments of the invention will be described with reference to the
accompanying' 90 2 780,050 drawings in which:
Fig. 1 is a fragmentary view in longitudinal section of a travelling
wave tube incorporating an improved radio frequency coupling means at
the ends of the helix in accordance with the principles of this
invention and having an electron gun arranged as stated above; Fig. 2
is a fragmentary view in elevation partially in section as viewed
along line 2-2 of Fig. 1; Fig. 3 is a fragmentary view in longitudinal
section showing another form of radio frequency coupling means; and
Fig. 4 is a view in elevation partially in section as viewed along
line 4-4 of Fig. 1.
Referring to Figs. 1 and 2 in the drawing, the pertinent parts of a
travelling wave tube are shown together with an illustration of an
embodiment of the radio frequency coupling means and electron gun
arrangement of the present invention. The travelling wave tube
generally comprises an electron gun unit 1 for producing a beam of
electrons for flow axially of the tube to a collector 2. The electron
gun unit is either separated from or made contiguous to an end plate 3
which is positioned crosswise envelope 4 which is composed of ceramic
material. Carried on the outer surface of envelope 4 is either a
permanent magnet or magnetic coil 5 which serves to preduce a magnetic
field axially of envelope 4. At the rear of the envelope is a second
end plate 6, both of the plates 3 and 6 being of magnetic material
having good conductivity. Interposed between these two plates 3 and 6
is an interaction section 7 including a helix 8 supported by
dielectric rods 9 or by a dielectric tube, whichever is desired. Any
suitable dielectric, such as glass or quartz may be employed. The rods
or tubing, as the case may be, are supported on the plates 3 and 6 in
openings 10 and 11, respectively. The helix 8 is connected between the
input and output r-f terminal connections 12 and 13 by means of radio
frequency coupling means 14 and 15, - respectively.
The coupling means 14 comprises a parallel line or waveguide
arrangement which includes plate 3 as one of the conductors and a flat
conducting strip 16 as the other conductor.

4. Strip 16 is separated from the parallel planar surface of plate 3 by
dielectric material 17 cf ceramic and is bonded to the solid
dielectric material The flat strip 16 has a length of approximately
one quarter wavelength or an odd multiple thereof at the operating
frequency of the device. For further disclosure of this general type
of transmission line, reference may be made to our specification Nos.
708,601 and 721,806. This type of strip conductor, spaced a small
fraction of a auarter wavelength from the parallel planar surface of
the conducting plate 3, provides a form of parallel waveguide for
propagation of electromagnetic waves from the coaxial connection 12 in
a mode closely simulating the TEM mode. The strip 16 is preferably of
a width sufficient to provide satisfactory coupling to the coaxial
line feeder and includes a transition piece 18 in coupled relation
with the vacuum portion of strip 16 at a high voltage point thereon.
Piece 18 is tapered as indicated in Fig. 2 to provide a gradual
transition from the strip width down to the conductor size forming the
helix. The space dimension h between strip 16 and the planar
conducting surface of plate 3 to cooperate in matching the impedance
of connection 12 to the impedance of line 8 may be obtained from the
expression Z h2i 2h log 80 d x774e i rd c e2) where Z is the
characteristic impedance of coaxial line, d is the width of strip 16
at the point of coupling to the coaxial line 12, h is the height of
strip 16 above the surface of plate 3, and e is the dielectric
constant of the dielectric material between the planar conducting
surface 3 and the strip 16.
The coupling means 14 may be considered as an impedance transformer.
Strip 16 is provided with a flanged portion 21 which provides90 a
short circuit between the planar conducting surface of plate 3 and
strip 16. Flange 21 locates a voltage null or zero impedance point and
is disposed approximately a quarter wavelength from the point of
coupling between 95 strip 16 and the helix 8. The impedance at this
coupling point would be equivalent to the high impedance of the helix
and the impedance along the matching section would vary in a given
manner between this high impedance 100 point and the zero impedance
point. At a point along section 14, the low impedance line 12 is
coupled at an appropriate point along section 14, consistent with the
low impedance of line 12, by means of bridge member 20 for coupling
centre conductor 19 to strip 16. Such a matching section provides a
low voltage standing wave ratio and accomplishes a broad frequency
match between the characteristic impedances of the two transmission
lines in 110 question. The coupling means 14 extends from the
atmospheric surroundings of envelope 4 into the vacuum portion of
envelope 4. The radio frequency energy is coupled from strip 16 to the
helix 8 through means of transition 115 piece 18 which reduces the

5. conductor in size from the width of strip 16 to approximately the
diameter of the wire forming helix 8.
The radio frequency coupling means 15 is identical to the form shown
at 14, the Dropagation therealong, however, being in the reverse
direction over the coupling means.
Envelope 4 is placed in contact with conductIng strip 16 and sealed to
the dielectric material 17 and strip 16 to insure a vacuum tight bond.
125 780,050 means such as spring fingers 35 (Fig. 1) pressing against
flange portions 34 to which the strips 31 are extended.' The disc 30
with the strips 31 are sealed to the ceramic tube 33 by wetting the
end of the tube 33 and the disc 70 together with the strips 31 with
glazing material and fusing these members together to form a vacuum
tight bond.
While Fig. 4 illustrates only three dc connections, it is obvious that
the number of 75 connections may be increased and appropriately
positioned on the face of disc 30 to accommnodate the connection of
dic potentials to any desired number of electrode elements.
The utilization of connecting strips 31 in 80 coupling potential to,
the various electrodes have been found to increase the power handling
capacity of this arrangement over a stem press employing round wires
of equivalent volume. For example, employing strips 31 85 having a
width of " and a thicknlmess of.005", the power handling capabilities
are increased over a round wire having an equivalent volume.
This increase of power handling capabilities is provided by the larger
cooling area for each of 90 the strips. If the number of conducting
strips placed upon the base 30 cause the spacing of the strips to
limit the potential difference that can exist between the strips, it
is possible to ridge the ceramic base between the strips as 95 is now
the practice in a conventional stem press to, increase the potential
difference required for breakdown.
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* 5.8.23.4; 93p